Devices that eject ink to generate images are well known. Throughout the life cycle of these devices, the image generating ability of the device requires evaluation and, if the images contain detectable errors, correction. Before such an imaging device leaves a manufacturing facility, the device should be calibrated to ensure that images are generated by the device without perceptible faults. As the device is used, the device and its environment may experience temperature instabilities and other conditions that may cause components of the device to expand and shift in relation to one another. As the device is used, the intrinsic performance of the device may change reversibly or irreversibly. Consequently, the image generating ability of such a device requires evaluation and adjustment to compensate for the changes experienced by the device during its life cycle. Sometimes these evaluations and adjustments are made at time or usage intervals, while at other times the adjustments are made during service calls made by trained technicians.
In ink jet imaging devices, ink jets expel ink through the response of piezoelectric actuators to firing signals generated by a printhead controller in the device. During the life cycle of the imaging device and from time to time during operation of the device, an ink jet may fail to expel one or more ink drops. In some instances, these missing ink jets may return to operational status without intervention. In other instances, one or more purge cycles may be required to return an ink jet to operational status. A purge cycle may be performed by introducing pressurized air to a printhead to expel ink from ink jet openings. In other purge cycles, the ink in a printhead is allowed to cool so it solidifies and then heat is reapplied to the printhead to melt the solidified ink before pressurized air is applied to expel ink from the ink jets. Purge cycles interfere with printer operations as images cannot be generated with a printhead when ink is being purged from the printhead. Consequently, scheduling purge cycles for the purpose of restoring ink jets to operational status is an important consideration for keeping an ink jet imaging system available for printing.